Similar to a lockup agreement that requires some shareholders to hold on to their stock for a certain period, the RSUs for employees aim to prevent the market from being swamped with too many of a company's shares immediately after an IPO.

Back in August, on a day when some of Facebook's early investors and executives had their first chance to sell their shares of the company, shares of Facebook fell 6%.

Though Facebook's employees are finally getting their chance to cash out, they won't be able to pocket all the earnings from their Facebook stock, as the IRS taxes RSUs as ordinary income on their full market value as of the day they vest.

Employers are required to withhold taxes when they settle RSUs. For many at Facebook -- whose windfalls can easily reach into the millions -- that will mean paying taxes at the top income tax rate. That's 35% this year for federal taxes. California, where most Facebook employees live, levies an additional 10.3% tax on individual income over $1 million.

Facebook says the tax rates will average around 45%, so it plans to withhold a significant portion of its employees' shares to cover the bill. Instead of selling those shares on the open market, Facebook will hang on to them and dip into its own cash stash and credit lines to pay the estimated tax bill of nearly $2.3 billion. The maneuver essentially functions like a stock buyback and reduces Facebook's outstanding share count.